Supporting structures for gas cooled core reactors built up from vertical columns are known. In such structures, either a number of supporting plates of polygonal configuration are arranged with lateral spacing adjacent to each other in a plane. Each of the plates are designed to carry a limited number of moderator columns, or else each moderator column is supported by a metal cylinder. At the same time, cooling gas channels present in both the moderator columns and the metal cylinders are aligned with each other.
An example of a supporting structure of the type described in the foregoing is shown in the West German published application No. 11 77 751. In this patent application, seven moderator columns, equipped with central borings, are associated with every supporting plate. The borings are coaxial with borings in the supporting plates. Only the column arranged in the center of the group of moderator columns is connected with the supporting plate. In this manner, the supporting plate is capable of expansion around the axis of this column.
In West German published application No. 1 122,641, a supporting structure is illustrated in which, as previously described, for every moderator column a supporting element in the form of a metal cylinder is provided. The moderator columns and the metal cylinders rest on each other on spherical bearing surfaces in the manner of a ball joint.
It is known from West German published application No. 1 194 071, that support of the moderator structure is made of a solid material of a nuclear reactor on a flat supporting surface. The supporting surface is composed of several parts, which are rigid in themselves. Each part of the supporting structure is carried by a certain number of support posts, arranged symmetrically around the axis of the moderator structure. The supporting surfaces surround a center part of the plate and several concentric annular frames which may have a small amount of free radical play between them.
According to the West German patent application No. 1 614,684, the supporting structure for a reactor block may consist of a cell structure composed of tubular elements, connected with each other at their ends by two horizontally arranged disks. Each of the tubular elements, which have hexagonal cross sections, is aligned with one of the columns of the reactor block. The supporting structure also serves as a biological shield, for which purpose at least two layers of graphite inserts are provided in the tubular elements.
Another supporting structure for the core of a gas cooled nuclear reactor is described in the West German patent application No. 1 956 266, in which the core rests on the bottom of the reactor pressure vessel upon refractory material. Cooling gas is admitted to the core through channels formed in the refractory material and connected with gas channels in the core and the gas space outside the core. The state of the art also includes a support floor for a pebble bed reactor consisting of pebbles of a high temperature material and a supporting structure for the weight of the pebble support layer and the fuel pebbles, the pebbles being piled directly onto the pebble support layer. The supporting structure and the pebble support layer are separated by a layer of tiles resistant to elevated temperatures.
Several vertical tubular stacks are provided at regular intervals for the constant venting of the fuel pebbles through the supporting structure and the pebble bed layer. These tubular stacks determine the least thickness and the average thickness of the supporting pebble bed.
In another pebble bed reactor, the THTR-300 MWe, the supporting floor for the bed of fuel pebbles consist of a plurality of hexagonal graphite blocks arranged into freely movable columns and having axial borings for the cooling gas. The columns formed by the graphite blocks are individually supported by one round column each, said round columns being attached into the floor and consists of graphite plates. The fixed point of the round columns is represented by the central pebble vent tube. By reducing the nominal dimensions of the hexagonal graphite blocks, expansion gaps are created which permit unhindered thermal expansion within the supporting floor without exceeding its overall dimensions. Under certain non-stationary operating conditions, e.g., in the case of accidental disruption, the gaps may add up and lead to relatively large single gaps. The closing of such gaps by relocating the graphite blocks requires high relocating forces which, however, in view of the dimensions of THTR-300 MWe are of minor importance. If the capacity of the reactor is increased, together with the dimensions of the reactor core and its installations, supporting floors designed on the principle described in the foregoing cannot be utilized without further changes, because the expansion gap and the relocating forces in the hexagonal arrangement of the blocks assume orders of magnitude, as a result of the substantial variation of parameters in the presence of which the supporting floor can no longer perform its function (vertical support of fuel elements, venting of fuel elements from the core, conduct of gas, shielding). A decisive factor in this respect is the flow behavior of fuel pebbles through the reactor core. A balanced flow behavior requires the availability of several pebble removal tubes, for which conical pebble inlets must be provided. Other parameters to be considered are stresses generated by dead weight, pressure gradients and the forces of absorber rods, also thermal expansions occurring with large dimensions of the core. All of these must be controlled.